In general mechanics, and in many types of applications covering the most different fields of technique, there are known to be screw and nut mechanisms for motion transmission. These mechanisms generally use constant pitch screws. In some cases, however, when special motion laws have to be followed, the mechanisms adopted are of the type with variable pitch screws.
A particular type of mechanism of this kind is that already adopted since some years to obtain the forward motion of weft carrying members (grippers or like) in the shed of shuttleless weaving looms (or looms with continuous weft feed). It is a mechanism wherein the reciprocating rotary motion of a gearwheel controlling the movements of the grippers carrying strap is obtained thanks to the similar motion of a variable pitch cam screw which passes through a slider alternately moving along a rectilinear path, means being provided to engage the slider with the screw, so as to produce the rotation of this latter in consequence of the rectilinear motion of the first, and viceversa.
In the constructions known up to date, said means for engaging the slider with the screw have always consisted of rolling members, especially of rollers carried by the slider and mating with the threading of the cam screw. This solution, which has provided satisfactory results and has allowed to considerably improve the performances of shuttleless gripper looms, has recently proved inadequate on account of the ever rising requirements to increase the speed of looms. Wear was in fact becoming prohibitive at the increased running speeds of the machine, probably due to the reduced rolling, or lack of rolling, of said rollers on the screw threading above certain speeds.
The studies made by the Applicant to overcome this drawback have discovered that, by replacing the known rolling means (having very reduced surfaces to bear on the screw threading) with oscillating sliding blocks having a suitable profile (adapted to mate with the screw threading, in correspondence of very wide bearing surfaces), it has been possible to surprisingly solve any problem connected with the speed increases of the machine, reducing wear--even at very high running speeds of the mechanism--to fully acceptable values.